Liquid crystal displays are used in a variety of electronic applications, including portable computers, flat screen televisions, and vehicular and aircraft instrumentation. Most liquid crystal displays require an illumination source for backlighting the display, located on the side of the display opposite a viewer, in order to effectively communicate the information on the display to the viewer.
Of concern to the liquid crystal display backlighting designer are the efficiency and brightness of the illumination source/liquid crystal display system. Accordingly, the liquid crystal display backlighting designer must arrive at the best compromise of brightness and efficiency for a backlighting system for a liquid crystal display. Of course, size and cost limitations are also of concern to the designer.
Backlighting systems for liquid crystal displays typically take one of two forms, as shown in prior art FIGS. 1 and 2. A first prior art design 10 is shown in FIG. 1. A pair of fluorescent lamps 12 and 14 shine through a light-blocking layer 15 and a diffuser 16 to illuminate a liquid crystal display 17. The light-blocking layer 15 and the diffuser 16 diffuse the light output by the lamps 12 and 14 to provide a more even distribution of illumination across the display 17. A reflector 18 having a reflective surface facing the lamps 12 and 14 redirects light emitted by the lamps, in directions other than toward the display, to improve system brightness and efficiency.
A disadvantage of prior art backlighting systems such as that shown in FIG. 1, however, is their overall thickness. Therefore, the system of FIG. 1 is inappropriate for use in compact environments. Also, because the light sources 12 and 14 in the system 10 are disposed directly behind the liquid crystal display, opposite the side of the display facing an observer, the light blocking layer 15 and the diffuser 16 must provide sufficient diffusion to eliminate hot-spots which would otherwise appear on the portions of the display directly in front of the lamps. The diffusion provided by these elements generally reduces the light transmission by approximately 20% to 50%, which adversely impacts the overall efficiency of the prior art system of FIG. 1, and significantly lowers the brightness of the backlight from the wall brightness of the fluorescent lamps.
A second type of prior art backlighting system 20 is shown in FIG. 2. The system 20 includes one or more light sources 22, 23, which illuminate the edge or edges of a light pipe 24. The light sources may be, for example, fluorescent lamps having light emitting apertures 25 located on the portion of the lamp facing the light pipe 24. The light pipe 24 provides illumination for a liquid crystal display 26, in effect, by redirecting light emitted by the fluorescent lamps 90.degree. toward the display. An index-matching material 27 may be provided between the light pipe 24 and the display 26. A reflector 28 may also be provided for directing light toward the display 26.
Backlighting a liquid crystal device by means of edge lighting a light pipe as in FIG. 2 generally eliminates the dimensional (thickness) problems associated with the direct backlighting system of FIG. 1. However, because the light which enters the narrow edge or edges of the light pipe must be spread uniformly over its length, edge lighting systems 20 such as that shown in FIG. 2 cannot provide the brightness that direct backlighting systems provide. Sufficient brightness is especially important in applications such as aircraft instrumentation panels which often need to be viewed in the presence of bright sunlight.
It is therefore an object of the present invention to provide a direct backlighting, high brightness, high efficiency device for illuminating an image plane such as a planar liquid crystal display, which includes an apertured fluorescent high brightness light source and a reflector for evenly distributing light exiting the aperture across a finite image plane. It is a further object of the present invention to provide such an illumination device having a low profile for use in confined-area applications. It is yet a further object of the present invention to provide an illumination system which may be constructed from a plurality of such devices to accommodate a variety of sizes of planar displays.